1. Field of the Invention
The present invention relates generally to electric motors and, more particularly, to a combined driver circuit for a brush motor and a switched reluctance motor.
2. Background of the Invention
Electric motors are used in a variety of applications. Two types of electric motors are brush motors and switched reluctance motors. Each has different operating characteristics and therefore, advantages suitable for different applications.
For example, a split brake system for a motor vehicle typically controls a front and diagonally opposed rear brake. The front brake requires high response performance and the rear brake requires significantly reduced response performance. Thus, a brush motor is used to actuate the rear brake and a switched reluctance motor is used to actuate the front brake.
With regard to the switched reluctance motor, the drive circuit is comprised of power switching devices and diodes for sequentially energizing the motor phase windings in accordance with the position of the rotor to produce a rotating magnetic field that interacts with the rotor poles to produce torque of a desired direction and magnitude.
A number of drive circuit topologies have been proposed to minimize the number of power devices in an effort to reduce the cost of the driver circuit. One such topology is described in U.S. Pat. No. 4,835,408 issued to Ray et al. The circuit disclosed in Ray, commonly referred to as a split-link circuit, utilizes capacitors to establish an intermediate voltage bus. One or more of the phase windings are coupled to the intermediate voltage bus. In automobile applications, the supply voltage is typically 12 volts. Such a design reduces the number of power devices required, but only one half of the supply voltage is available for energizing the phase windings. As a result, the efficiency of the drive is reduced due to increased switching and conduction losses.
U.S. Pat. No. 5,900,712 issued to Disser et al, maintains a second bus at a voltage which is either higher or lower than the supply voltage, thereby extending the voltage range of the supply.
With regard to the brush motor, motor direction is controlled by the polarity of the voltage applied to the brush terminals. Since it is desirable to maximize power to the brake, it is desirable to apply full supply voltage to the brush terminals.
The present invention is aimed at one or more of the problems above.
In one aspect of the present invention, a combined drive circuit for a switched reluctance motor and a brush motor is provided. The switched reluctance motor has multiple phase windings. The combined drive circuit includes a DC power source with first and second supply buses. A SR motor driver supplies current to the switched reluctance motor. The SR motor driver has a first capacitor coupled to the first and second supply buses for storing energy from the DC power source and a second capacitor coupled to the first capacitor and a third supply bus, thereby developing a third supply bus voltage across the second capacitor. A brush motor driver is coupled to the first and second capacitor. The SR motor driver is adapted to supply current to the brush motor. The brush motor driver has first, second, third and fourth switching elements coupled to the brush motor.
In another aspect of the present invention, a combined drive circuit for a split brake system of a motor vehicle is provided. The split brake system has a switched reluctance motor and a brush motor. The switched reluctance motor has multiple phase windings. The combined drive circuit includes a DC power source with first and second supply buses. A SR motor driver supplies current to the switched reluctance motor. The SR motor driver has a first capacitor coupled to the first and second supply buses for storing energy from the DC power source and a second capacitor coupled to the first capacitor and a third supply bus, thereby developing a third supply bus voltage across the second capacitor. A brush motor driver is coupled to the first and second capacitor. The SR motor driver is adapted to supply current to the brush motor. The brush motor driver has first, second, third and fourth switching elements coupled to the brush motor.